This invention relates to an epicyclic reduction gear type starter device which transmits the rotation of a DC motor through epicyclic reduction gear means to a pinion which is brought into and out of engagement of the ring gear of an engine, and more particularly to a ring-shaped internal gear adapted to the epicyclic reduction gear type starter device.
FIGS. 3 through 5 shows a conventional epicyclic reduction gear type starter device. The starter device 1 comprises: a DC motor 2; an over-running clutch 4 slidably mounted on an output rotary shaft 3 which is coupled to the armature rotary shaft of the DC motor 2; and epicyclic reduction gear means 5 for reducing the speed of rotation of the armature rotary shaft and transmitting the rotation thus speed-reduced to the clutch outer of the over-running clutch 4 through the output rotary shaft 3. The starter device 1 further comprises a shift lever 6 which is driven by an electromagnetic switch (not shown), so that a pinion 7 mounted on the output rotary shaft 3 is moved into or out of engagement with the ring gear of the engine.
The epicyclic reduction gear means 5 comprises: an internal gear 10 of resin coupled to a housing 9 which is secured to a front bracket 8; a spur gear 11 formed on the armature rotary shaft; and a plurality of planet gears 12. The planet gears 12 are supported by a carrier 13, which is coupled to the output rotary shaft 3. In FIG. 3, reference numeral 14 designates a packing.
As shown in FIG. 5, two inserting grooves 15 are formed in the outer periphery of the internal gear 10 in such a manner that they are positioned diametrically opposite. Through-bolts (not shown) are inserted into those inserting grooves 15. In addition, two protrusions 16 are provided diametrically opposite from each other and at positions which are circumferentially shifted about 30.degree. in a counterclockwise direction from the above-described two inserting grooves 15, respectively. The two protrusions 16 are locked to the housing 9 for prevention of the rotation of the internal gear.
The operation of the conventional epicyclic reduction gear type starter device thus constructed will be described.
When the DC motor 1 is electrically energized, the rotation of the armature rotary shaft is transmitted through the spur gear 11 to the planet gears 12. Thus, with the speed of rotation reduced by the epicyclic reduction gear means 5, the rotation is transmitted to the over-running clutch 4 and the output rotary shaft 3. In this operation, the pinion 7 engaged with the over-running clutch 4 is rotated, so that the ring gear of the engine engaged with pinion is rotated.
The above-described conventional epicyclic reduction gear type starter device suffers from the following difficulties: When a heavy load is applied to the epicyclic reduction gear means 5, the internal gear 10 is deformed polygonally in correspondence substantially to the number of planet gears 12, so that the engagement between the internal gear 10 and the planet gears 12 is shallow. As a result, the bearing pressure is increased, or the load applied to the tips of teeth increases the dedendum stress, so that the teeth are broken.
The through-bolt inserting grooves 15 are different in thickness from the locking protrusion 16. Hence, when the internal gear 10 is molded from resin, the resultant molding deforms; that is, the internal gear 10 formed is low in gear accuracy.